1. Field of the Invention
This invention relates to a multilayer film structure which can be used as a phase plate for phase contrast microscopes, a modulation filter for modulation contrast microscopes, etc.
2. Description of the Related Art
From the past, a multilayer film structure has been known which can be used as a phase plate for phase contrast microscopes, a modulation filter for modulation contrast microscopes, etc. The multilayer film structure of this type needs such properties that:
light transmitted through the multilayer film structure can be controlled to have a desired intensity, PA1 detrimental reflecting light is not produced, and the phase difference between light transmitted through the multilayer film structure and light not transmitted can be controlled to a desired value. PA1 (1) The multilayer film structure includes at least light absorption layers and transparent layers. PA1 (2) The transparent layer has the function of an antireflection film for cutting off rays of light reflected by the light absorption layer. PA1 (3) In order to satisfy the phase condition for the antireflection film, a product (nd) of a thickness (d) and a refractive index (n) of the transparent layer sandwiched between the light absorption layers is approximately 1/4 a wavelength (.lambda.) of light intended for antireflection. PA1 (4) In order to satisfy the amplitude condition for the antireflection film, the refractive index and absorption coefficient of the light absorption layer have such a relationship that a value R becomes small which is given by EQU R=(n-n0).sup.2 +k.sup.2 /(n+n0).sup.2 +k.sup.2 PA1 (5) The phase change of transmitted light caused by the light absorption layer is compensated by the transparent layer. PA1 (6) the phase difference between light transmitted through the multilayer film structure and light not transmitted is brought to a desired value.
In order to secure these properties, it is necessary to meet the following requirements:
where no is the refractive index of a substance constituting the transparent layer, n is the refractive index of a substance constituting the absorption layer, and k is the absorption coefficient of the substance constituting the absorption layer.
For the film structures taking account of requirements (1) to (4), the technologies are known which are disclosed in Japanese Patent Publication Nos. Sho 52-29935 and Sho 55-47361. As for requirements (5) and (6), the technology is known which is set forth in Japanese Patent Preliminary Publication No. Sho 56-8107.
Publication No. 52-29935 uses Ti and Inconel to control the spectral transmittance of the film, but fails to disclose the means of controlling the spectral phase difference and the spectral reflectance thereof. Publication Sho 55-47361 is designed to control the spectral reflectance of the multilayer film structure by alternately building up the transparent layer and the light absorption layer, and uses Ti and Cr for the light absorption layer. This prior art, however, does not disclose the means of controlling the spectral phase difference of the film structure. Further, Publication Sho 56-8107 is constructed to alternately laminate the light absorption layer and the transparent layer so that the antireflection of the light absorption layer is performed, but does not set forth the means of controlling the spectral phase difference.
In short, the multilayer film structures of the prior art so far discussed have no means of bringing the spectral phase difference to a desired value, and are not designed to control the spectral phase difference, spectral transmittance, and spectral reflectance at the same time. Thus, even though the film structure is built to bring the phase difference, transmittance, and reflectance to proper values with respect to a reference wavelength, they will be deviated from the proper values with respect to other wavelengths. Consequently, in the phase contrast microscope, for example, a good image cannot be obtained over the entire visible wavelength region.
On the other hand, it is known that since the multilayer film structure per se is subjected to a stress, which may cause cracks in the film, the combination of substances bringing about compressive and tensile stresses relieves the stress of the entire film to prevent the cracks from being produced (Fujiwara, "Optical Thin Film", Kyoritsu Shuppan K. K.). However, when the number of layers increases and the thickness of the whole film becomes greater, the possibility of stress cracking will be higher. In Publication Sho 55-47361 mentioned above, metallic layers composed of Ti or Cr and transparent layers of MgF.sub.2 are built up as the multilayer, so that there is the possibility that cracks are produced.